A smile seems the most natural of emotional expressions. We smile easily and often unthinkingly; babies smile; it is, as Colin Jones notes in his introduction to this book, ‘the most banal and unremarkable of social gestures’. Or is it? It might be an easy expression for others to recognise, as the work of psychologist Paul Ekman demonstrated in the 1970s, but does this necessarily make it a ‘natural’ expression that invariably denotes happiness?

It is with this intriguing question that Jones sets out to discover what else lies behind the seemingly ‘banal’ smile. The Smile Revolution in Eighteenth Century Paris beautifully complicates the notion that the smile is a static and timeless form of emotional expression – a gesture that signifies sincere and ‘true’ feeling. Indeed, it is ironic that the ‘true’ smile – the mouth raised by the zygomatic major muscle of the cheek, accompanied by movement of the muscles around the eyes – is also called the ‘Duchenne smile’ after physiologist Guillaume Duchenne de Boulogne. The smiles that Duchenne produced by attaching electrodes to a man’s face in order to stimulate his unfeeling muscles and photograph the results are certainly not ‘natural’ smiles in the emotional sense, though they may have been a perfect demonstration of the physiological.

Years before the experiments of Duchenne, however, the smile – sincere or otherwise – was already causing a stir in France. Focusing on the French capital, Jones charts how the ‘snooty, aggressive, closed-mouth smile’ of c.1700 came to be usurped by the ‘open-mouth, white-tooth smile’ by the 1780s. It was a development dependent upon several practical factors – the growth of modern dentistry, new consumer goods, and technical skill – but equally upon ideas about respectability, propriety, and the appropriateness of emotional expression. Turning to chapter one, ‘The old regime of teeth’, we are met with Louis XIV staring out at us from Hyacinthe Rigaud’s 1701 portrait of the Bourbon king. He is resplendent in his robes, affecting a somewhat cocky, swaggering stance, yet when one looks closely at his face the illusion begins to break down, for Louis’s ‘hollow cheeks and wrinkled mouth reveal a ruler with not a tooth in his head’. Even the wealth of the royal court couldn’t save the king’s teeth (and smile) from the same fate as that of his subject’s. A lack of dental care, coupled with malnutrition in the poorer sections of society, meant that you were lucky to have a tooth left in your head in your later years. Jones paints a vivid picture of this experience: the loss of one’s looks, problems with eating, and an embarrassing inability to articulate one’s words properly. Thus, the cavernous, toothless mouth or line of rotting teeth was kept hidden – smiling was not a pleasant experience for the smiled-at.

The smile also demonstrated something less tangible at this time: a lack of control, an absence of comportment, even some form of mental ineptitude. Jones’s wonderful term ‘facial regime’ conveys the sense in which emotional expressions may be ‘policed’ according to prevailing cultural mores. Early 18th-century France took this policing especially seriously, it seems, with books such as Jean-Baptiste de La Salle’s Rules of Decorum and Christian Civility (1703) advising the reader against indecorous emotional displays such as smiling and laughing (a notable strength of Jones’s volume is the analysis of smiles as depicted not only in art, but also literature). ‘God would not have given humans lips’, the argument went, ‘if He had wanted the teeth to be on open display’. In Western art dating back beyond the 18th century, the open mouth tended to signify ‘extreme passion’ or that the sitters were ‘plebeian or insane’, such as Jusepe de Ribera’s Club-footed Boy (1637). Only children or those lacking proper control of their emotions smiled openly.

By the mid-18th century, though, the tide was beginning to turn. With the rise of the ‘cult of sensibility’, emotional expression was no longer an unfortunate aspect of human existence but the very core of being human. Novels such as Samuel Richardson’s Pamela and Clarissa (1740 and 1748) included characters who smiled ‘enchantingly’ or ‘sweetly’, transforming the gesture into a romantic rather than a grotesque one.

The smile of sensibility was also a smile of serendipity, however. Throughout The Smile Revolution, Jones expertly weaves together these discussions of cultural and social feeling with the broader technical and professional developments that went alongside them and, in many cases, made them possible. The relative scarcity of published histories of dentistry makes Jones’s book an important contribution to the history of medicine as well as the history of the emotions, for the dentist is revealed to be a key facilitator of the ‘smile revolution’. If the prospect of a trip to the dentist in the present day terrifies you, then spare a thought for the patients of Le Grand Thomas, tooth-puller extraordinaire and a familiar sight to the Parisian pedestrian. Tooth-pulling (for this was almost the sum total of dental treatment available to the early 18th-century citizen suffering from toothache) was a public activity and, as Jones highlights, a distinctly performative one. Thomas operated from a covered cart on the Pont-Neuf, accompanied by two musicians playing trumpet and violin. If this all sounds rather jolly, it was no doubt counteracted by the formidable presence of Thomas himself (not nicknamed ‘Grand’ for nothing) and the disconcerting slogan on his flag: ‘the tooth, and if not, the jaw’.

By mid-century, Thomas and his counterparts were facing a new threat from a more self-consciously scientific band of specialists, the chirurgien-dentistes. Under the care of these new medical men, patients were sympathised with rather than made a spectacle of, as notions of sensibility inspired the relocation of dentistry from the street to the private consulting room, and demonstrated a concern for the fears of patients, with instruments kept as far out of sight as possible. Here, Jones introduces an array of fascinating developments, showing how the smile – once an unthinkable gesture for the cultured Frenchman or woman – became a marketable, and manufactured, commodity. As medical advertising expanded in the latter part of the century, toothpicks, tongue-scrapers, toothbrushes, gargles, and lipsticks (to enhance the whiteness of the teeth) transformed the interior of the mouth from a dirty secret to an advert for one’s own conspicuous consumption (this worked both ways, of course, if one over-indulged in sugary confectionery).

This, then, was the smile revolution of Jones’s title. It was a revolution that didn’t last, however, and it is in his exploration of the multiple meanings of the smile that Jones most successfully drives home how difficult it is to conceive of a single emotional expression as an entity with a distinct meaning. Part-way through my reading of this book, Paris suffered a new upheaval when an attack was launched on the offices of satirical newspaper Charlie Hebdo, leaving 12 people dead. In the aftermath of the attack, the former editor-in-chief of the newspaper, Philippe Val, cast the laughing mouth as more than simple emotional expression: ‘We have laughed so much, we must continue to laugh – it is difficult today but it’s the absolute weapon’. The Parisian smile was an act of defiance, the laugh a political statement.

The defiant, serious, uses of the smile were apparent in late eighteenth-century France, too, as one revolution was interrupted by another. During the French Revolution, as Jones puts it: ‘The open mouth that many would come to associate with the Revolution was not the white-tooth smile; but rather the gaping, gothic mouth set in a scream of violence’. The sweet or enchanting smile of sensibility was suddenly inappropriate and unwanted, and the open mouth – ‘a quintessentially Rabelaisian orifice’ – to be firmly closed once more, even if it resisted with its screams. In the context of the Terror, the smile took on a sinister edge: the ‘scaffold smile’ in the face of the executioner was ‘an emblem of political resistance that could unsettle those most hardened to violence. Executioner Sanson witnessed the scaffold smile several times, and found it disturbingly touching: ‘I could become accustomed to the horror that my presence occasioned, but it was altogether more difficult to take to the guillotine individuals ready to thank me for doing so’.

In the final years of the 18th century, the smile was both metaphorically and literally wiped from the faces of many Parisians, as the severed heads of enemies of the revolution were publicly displayed and mutilated, mouths stuffed with straw. The Terror also presented macabre opportunities for studying the mechanics of facial expression. The head of Charlotte Corday, Marat’s assassin, was said to have blushed after being slapped in the face by the executioner, and tales circulated of experiments in which the heads of victims were re-attached after death.

Even without these dark details, it is difficult not to picture the grinning skull throughout Jones’s book. In placing the white-toothed smile, or toothless leer, centre stage, we have a sense of why past commentators might have found the display of teeth so disturbing, rotten or otherwise. In looking at the teeth, we look at the skull – a gaze into the interior depths of the body that would, with any other body part, be transgressive. The parting of the lips in a smile reveals the skeleton beneath, a reminder of one’s own mortality (or indeed someone else’s, should you be the lucky recipient of transplanted teeth, which were suspected to come from morgues and cemeteries). By the end of the century, the skull and teeth were also thought to afford a glimpse of something much deeper, and in his chapter ‘The Transient Smile Revolution’ Jones sets his discussion of the Terror alongside broader developments in scientific theory. Johann Caspar Lavater’s work on physiognomy was less interested in facial expressions than the underlying structure, reading character from teeth and skulls. In this more serious era of scientific pursuits, the smile again became something frivolous and unimportant.

Jones’s choice of the 18th-century French capital as a route into the history of an emotional expression as widely recognised as the smile may seem a narrow one at first sight. It quickly becomes clear, however, that it is the perfect place: one in which fashion – both in clothing and behaviour – was paramount, where ideas were first energetically circulated, then rigorously policed. That the smile was something peculiarly ‘Parisian’ at this time is alluded to in an anecdote about the Scottish physician Tobias Smollett, who was irritated by his endlessly smiling hosts during a visit to Paris in the 1760s. If such stark differences could be detected between two countries relatively close to one another in the same decade (and certainly others too commented on the smiling French and the morose English), we are again led to consider how emotional expression may be a culturally-bounded rather than entirely natural phenomenon.

The ‘Postscript’ reinforces the point that emotional expression can also depend on available technologies: the open-mouthed smile was rarely in evidence in the late 19th century as long sitting times for photographs risked transforming the smile into an unpleasant rictus. New camera technology in the 20th century, though, brought the smile back to life and has rendered it apparently immortal in western culture. It is here, perhaps, that it has become the banal gesture that Jones refers to in his introduction. If the facial regime of early 18th-century France was to keep one mouth’s firmly closed, the regime of the present seems quite the opposite. On a camera being produced, we are ordered to ‘Smile!’; and that the smile may be a performative gesture apparently devoid of emotion was evident in the trend for ‘funeral selfies’ that was roundly mocked on the internet throughout 2014.

Happily, Jones raises a smile or two himself with his playful descriptions peppered throughout the book: of the toothbrush as ‘talismanic technology of the Smile Revolution’, or of older views of the body as ‘a kind of clumsy container in which humoral fluids sloshed around’. It is immensely readable, drawing on a range of sources (paintings, satirical cartoons, scientific illustration, novels) that highlight not just the evolution of a facial gesture but the rise of professional medical specialization, 18th-century consumption patterns, and the dissemination of fashions. In his introduction, Jones describes the smile as ‘transient, slippery, and short-lived’, yet by the end of the book he demonstrates his own grip on the smile to be a firm one in a volume that will surely have enduring significance.